There are conventional devices for the active power factor correction (PFC) for forced switching power supply units used in commonly used electronic devices such as computers, televisions, and monitors, and for supplying fluorescent lamps, that is forced switching pre-regulation stages whose task is to absorb a nearly sinusoidal current from the mains which is also in phase with the mains voltage. Therefore a forced switching power supply unit of the present type comprises a PFC and a converter of direct current into direct current (or “DC-DC converter”) connected to the output of the PFC.
A forced switching power supply unit of the traditional type comprises a DC-DC converter and an input stage connected to the electricity distribution mains constituted by a full-wave rectifying diode bridge and a capacitor that is connected immediately downstream so as to produce a non-regulated direct voltage starting from the sinusoidal mains alternating voltage. The capacity of the capacitor is so big that at its terminals there is a relatively small ripple in relation to the continuous level. The rectifying diodes of the bridge, thus, will only conduct for a small portion of each half cycle of the mains voltage, given that the instantaneous value of this is lower than the voltage on the capacitor for the majority of the cycle. Thus, the current absorbed by the mains will be constituted by a series of narrow pulses whose amplitude is 5–10 times the resulting average value.
This presents considerable consequences: the current absorbed by the line has much greater peak values and effectiveness in comparison to the case of absorption of sinusoidal current, the mains voltage is distorted by effect of the almost simultaneous impulsive absorption of all the users connected to the network, in the case of three-phase systems the current in the neutral conductor is greatly increased, and there is a low use of the energetic potentials of the electricity producing system. In fact, the pulse current waveform has abundant odd harmonics which, even though they do not contribute to the power delivered to the load, contribute to increase the effective current absorbed by the mains and thus to increase the dissipation of energy.
In quantitative terms all this can be expressed both in terms of Power Factor (PF), intended as a ratio between the real power (that which the power supply unit gives to the load plus that dissipated within in the form of heat) and the apparent power (the product of the effective mains voltage and the effective current absorbed), and in terms of Total Harmonic Distortion (THD), generally intended as a percentage ratio between the energy associated with all the higher order harmonics and that associated with the fundamental harmonic. Typically, a power supply unit with a capacitive filter has a PF between 0.4–0.6 and a THD greater than 100%.
A PFC, placed between the rectifier bridge and the input of the DC-DC converter, permits the absorption from the mains of a nearly sinusoidal current that is also in phase with the voltage, making the PF near 1 and reducing the THD.
FIG. 1 shows a pre-regulator stage PFC comprising a boost converter 20 and a control device 1. In this case the control device is a L4981A control device produced by STMicroelectronics S.p.A. The boost converter 20 comprises a full-wave diode rectifier bridge 2 receiving at its input a mains voltage Vin, a capacitor C1 (that acts as a filter for the high frequency) having its terminals connected to the terminals of the diode bridge 2, an inductance L connected to a terminal of the capacitor C1, a MOS power transistor M having its drain terminal connected to a terminal of the inductance L that is downstream of the inductance and having its source terminal connected to ground, a diode D having its anode connected to the common terminal of the inductance L and the transistor M and its cathode connected to another capacitor Co, which has its other terminal connected to ground. The boost converter 20 generates at its output a direct voltage Vout on the capacitor Co which is greater than the maximum mains peak voltage, typically 400 V for systems supplied with European mains or with universal supply. This voltage Vout is the input voltage of the DC-DC converter connected to the PFC.
The boost converter also comprises a detecting resistor Rs that is connected between the source terminal of the transistor M and a terminal of the diode bridge 2 that closes the circuit permitting the reading of the current that flows through the inductor L.
The control device 1 must keep the output voltage Vout at a constant value through a feedback control action. The control device 1 comprises an operational error amplifier 3 for comparing a part of the output voltage Vout, that is the voltage Vr given by Vr=R2*Vout/(R2+R1) (where the resistors R1 and R2 are connected in series and in parallel with the capacitor Co), with a reference voltage Vref, for example of 2.5V. The error amplifier 3 generates an error signal Se that is proportional to their difference. The output voltage Vout presents a ripple at a frequency that is double that of the mains and superimposed onto the direct value. If however the band amplitude of the error amplifier is considerably reduced (typically lower than 20 Hz) through the use of a suitable network of compensation comprising at least one capacitor and assuming almost stationery regular functioning, that is with constant effective input voltage and output load, this ripple will be greatly attenuated and the error signal will become constant.
The error signal Se is sent to a multiplier 4 where it is multiplied by a signal Vi given by a part of the mains voltage rectified by the diode bridge 2. The multiplier 4 also receives a signal output from an inverter-squarer block 41 whose input receives a voltage signal Vrap, which is representative of the effective value of the mains voltage obtained through a block 42; the signal output from the block 41 is 1/Vrap2.
At the output of multiplier 4 there is a current signal Imolt given by a rectified sine curve whose amplitude depends on the effective mains voltage and the error signal Se. The current signal Imolt flows through the resistor Rm and generates a voltage that represents the sinusoidal reference for the modulation PWM. The voltage signal is input to the non-inverting terminal of an operational amplifier 6 whose inverting input is grounded. With IL the current that flows on the resistor Rs for the virtual ground principle we have the following.
      I    L    =            Rm      Rs        ⁢    Imolt  Thus, the current IL will evolve as a rectified sine curve.
The signal output from the operational amplifier 6 is input to the inverting terminal of a PWM comparator 5 that has its non-inverting terminal connected to an oscillator 7 that supplies a saw-tooth signal whose frequency determines the working frequency of the pre-regulator.
If the signals input to the comparator 5 are equal, the comparator 5 sends a signal to a control block 10 for driving the transistor M and which, in this case, turns it off. A filter positioned at the input of the stage eliminates the switching frequency component and ensures that the current absorbed by the mains has the form of the sinusoid envelope. Another signal output from the oscillator 7 is constituted by a series of pulses in correspondence with the trailing ramps of the saw-tooth signal; the signal is the set input S of a set-reset flip-flop 11, which has another input R that receives the signal output from the comparator 5 and has an output signal Q. The output signal Q is input to a driver 12 that commands the turn on or the turn off of the transistor M.
As long as the boost converter functions correctly, the voltage generated at the output must always be greater than the input voltage. In its most typical embodiment, in a pre-regulator PFC the output voltage is set around 400V so as to be greater than the mains peak voltage in all its interval of variation (from 124.5 to 373.4 V in the case of universal supply). In another embodiment known as “boost follower” or “tracking boost”, the output voltage is regulated at a value, depending on the effective input voltage, nevertheless always greater than the peak voltage.
The “tracking boost” approach presents some advantages: at equal frequency the inductance of the inductor of the boost converter is lower, the same as the effective current in the low line voltage MOSFET. This approach is meeting with growing success, especially in mains adaptors for high level notebooks because it makes it possible to use a smaller boost inductor and because the overall efficiency is better. Nevertheless a boost converter of this type presents some disadvantages, such as an increase in the losses in the output diode, the need for a larger output capacitor, and the impossibility of optimizing the DC-DC converter downstream for a constant input voltage.